1. Field of the Invention
The present invention relates to a method of photometric in vitro determination of the content of an analyte in a sample located in a measuring chamber device with a measuring chamber which has a defined radiation path length and has one at least partially transparent wall part, said measuring chamber being in optical communication with an optical system adapted for the analyte and comprising a radiation source and a radiation detector.
2. Description of the Related Art
In photometric analysis of the analyte content in a sample located in a measuring chamber, it is in certain cases desired, for example in connection with blood measurements, to make the measuring chamber wall parts located in the radiation transmission path very thin. This may for example be required if the wall parts of a measuring chamber are made from a material significantly absorbing the measuring radiation. In certain instances the thin wall parts may, however, give rise to errors in the determination of the analyte content. E.g. this situation will arise when the radiation wavelength is comparable to the optical thickness of the measuring chamber walls, said optical thickness being defined as the physical thickness of the walls multiplied by the refraction coefficient of the wall material. Thus, the problem is mainly connected with long wave measuring radiation such as infrared or near-infrared radiation.
When the measuring chamber wall has smooth, plano-parallel surfaces, radiation will be reflected and/or refracted in the interfaces between the surroundings and the external walls of the measuring chamber and in the interfaces between the internal walls of the measuring chamber and the sample, respectively. The extent to which the radiation is reflected and/or refracted depends on the refraction coefficients for the wall material, the surroundings and the sample. Said reflected and/or refracted radiation gives rise to a sinusoidal interference spectrum which superposes the measuring spectrum. Particularly in cases where absorption caused by the analyte is relatively small, the above interference spectrum may dominate the measuring spectrum.
The interference spectrum is extremely dependent on the thickness of the walls, and even small variation in thickness will displace the spectrum considerably. Such small variation is almost impossible to avoid in automated mass-production without extremely great expense. Therefore, each measuring chamber device will have its own characteristic interference spectrum.
The effect of interference in connection with foils is disclosed in Hummel/Schou, "Atlas der Kunststoff-Analyse", 1968, ps. 64-65. In this reference a possible solution to the problem is also given; namely to provide one of the parallel sides with a fine roughness in order that the reflected radiation beams do not interfere, but are being scattered diffusely. However, this method is not very advantageous when a large number of uniform products are to be produced as the roughness must be very fine in order to remove interference completely. The roughening of the foil will result in an additional expensive manufacturing process step and further it is difficult to ensure that all products be provided with the same roughness and thereby with the same radiation transmission properties.